Commitments, contingencies, and guarantees Commitments, Contingencies, And Guarantees (Collateral pledged under repurchase agreements) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	$ 1,453	$ 1,461
Repurchase agreements	1,404	1,403
Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0	1
U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	245	240
Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	1,208	1,220
Repurchase Agreements [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Secured Borrowings, Gross, Difference, Amount	49	58
Maturity Less than 30 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	369	569
Maturity Less than 30 Days [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0	0
Maturity Less than 30 Days [Member] | U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	9	230
Maturity Less than 30 Days [Member] | Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	360	339
Maturity Greater than 90 Days [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	1,084	892
Maturity Greater than 90 Days [Member] | Cash and Cash Equivalents [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	0	1
Maturity Greater than 90 Days [Member] | U.S. Treasury And Agency [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	236	10
Maturity Greater than 90 Days [Member] | Collateralized Mortgage Backed Securities [Member]
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Assets pledged under repurchase agreements	$ 848	$ 881